This invention relates to a method of stabilizing the laser beam apparatus and an apparatus utilizing the method. More particularly, this invention relates to a method of stabilizing and controlling the wavelength and/or output power of laser beam.
Conventionally, narrow bandwidth laser emitting a specific wavelength beam is utilized in various research areas including high resolution atomic/molecular spectroscopy, isotope separation and semiconductor material technology because of the high wavelength fidelity of such beam.
Recently, efforts have been paid for utilizing a narrow bandwidth excimer laser as a lithography light source in the process of semiconductor production in the semiconductor manufacturing industry.
The properties of a laser beam generated and emitted from a laser apparatus are determined by the wavelength distribution of the gain of the laser medium that the apparatus comprises. A wavelength band showing a large gain normally has a narrow bandwidth, which is dependent to some extent on the properties of the laser medium used in the apparatus. However, for using a conventional laser apparatus as light source for spectroscopy or lithography, the bandwidth of the laser beam emitting from the laser apparatus must be narrower. A technique of disposing wavelength selecting devices such as grating, etalon, birefringent filter or prism within the laser resonator of a laser apparatus has been proposed to realize a narrow bandwidth laser light. This kind of wavelength selecting device works as an optical loss in the laser resonator in such a manner that the gain of the laser medium is limited to those wavelengths which are found within a very narrow bandwidth. It is a well known fact that the optical properties of a wavelength selecting device of this type is liable to be affected by environmental factors such as displacement due to mechanical vibrations and/or fluctuations in the atmospheric pressure and ambient temperature. Therefore, the output power of laser beam and/or wavelength of the laser beam can be varied without any changes in the state of the laser medium or its degradation.
On the other hand, narrow bandwidth laser apparatus to be used for the above applications is required to be highly and constantly stable in terms of output power and oscillation wavelength. It has been proposed for a laser apparatus to have a feedback system for controlling the output power and oscillation wavelength in an attempt to meet this stability requirement. With such a system, a part of generated laser beam is introduced into a spectroscopic system (e.g., an etalon spectroscope) and a beam intensity detector to detect the oscillation wavelength and output power respectively, which is fed back to the bandwidth narrowing devices for fine adjustment of the devices by controlling the variations taking place in the conditions of oscillation so that fluctuations in the oscillation wavelength and output power may be minimized.
Narrow bandwidth laser apparatuses equipped with a control system as described above are currently commercially available and find a number of practical applications. However, the complicacy of the signal processing system, untimely and delayed control operations, generation of overshooting phenomena and other problems of such apparatus make it very difficult to control and operate them in a stabilized manner.